The present invention relates to a laser optical system such as to be employed in a laser photo plotter, and more particularly to a laser optical system including a laser source that emits a laser beam having a plurality of wavelengths (i.e., a multiline laser).
The exposure device such as a laser photo plotter, which requires relatively large laser power, employs a gas laser. A laser beam emitted from a gas laser is deflected by a polygon mirror and is converged through an imaging optical system such as an fxcex8 lens to form a beam spot on an object surface to be exposed. The beam spot scans the object surface to form a scanning line as the polygon mirror rotates.
A gas laser generally emits a laser beam having a plurality of peak wavelengths. For example, an argon laser emits a laser beam having a plurality of peak wavelengths in ultraviolet and visible regions.
In order to keep high energy efficiency, it is desirable to use a plurality of peak wavelengths of a laser beam emitted from a gas laser.
However, since the minimum diameter of a beam spot on the object surface is proportional to a wavelength and is inversely proportional to a beam diameter incident on an imaging optical system, the beam spots formed by the different wavelengths are different in diameter when beam diameters incident on the imaging optical system are identical to one another.
Therefore, when the laser beam emitted from a multiline laser source is divided into a plurality of wavelengths and the divided beams form respective beam spots at different points to form a plurality of scanning lines per one scan, the widths of scanning lines become different depending on the wavelength. Further, when the beam spots of the respective wavelengths are formed at the same point on the object surface, a distribution of light quantity within the beam spot becomes irregular. In either case, a quality of an image formed on the object surface is degraded.
It is therefore an object of the present invention to provide a laser optical system capable of making the beam spots have the same diameter when the laser beam having different wavelengths is used to form the beam spots on the object surface.
For the above object, according to the present invention, there is provided an improved laser optical system, which includes a correcting optical system that adjusts a diameter of a laser beam incident on an imaging optical system such that the diameter of the incident laser beam decreases as the wavelength of laser beam becomes shorter.
Namely, the laser optical system of the invention includes a multiline laser source that emits a laser beam having a plurality of peak wavelengths, an imaging optical system that converges the laser beam emitted from the multiline laser source to form beam spots of respective peak wavelengths on an object surface, and the correcting optical system that makes.beam spots have the same diameter for at least two selected peak wavelengths.
The correcting optical system satisfies the following condition (1):                                                         λ              A                                      λ              B                                       less than                               ω            A            xe2x80x2                                ω            B            xe2x80x2                           less than                               (                                                            λ                  A                                                  λ                  B                                                      )                    3                                    (        1        )            
where xcexA and xcexB are the selected peak wavelengths and xcfx89xe2x80x2A and xcfx89xe2x80x2B are diameters of the laser beam to be incident on the imaging optical system at the selected peak wavelengths xcexA and xcexB.
Further, it is preferable that the correcting optical system satisfies the following condition (2):                                           ω            A            xe2x80x2                                ω            B            xe2x80x2                          =                                            λ              A                                      λ              B                                .                                    (        2        )            
In the specification, a xe2x80x9cbeam spotxe2x80x9d means a spot formed on the object surface by each wavelength. A laser beam having plurality of wavelengths forms a plurality of beam spots at the same point or different points.
With the above described construction, since the correcting optical system adjusts the beam diameters of the respective wavelengths, the diameters of the beam spots formed on the object surface can be the same in spite of the wavelength. When at least the condition (1) is satisfied, a difference in the spot diameters becomes insignificant in practical use. Further, when the condition (2) is satisfied, the spot diameters will be completely identical.
In order to give the above described characteristics to the correcting optical system, it is preferable to satisfy the condition (3):                                                         λ              A                                      λ              B                                       less than                               m            A                                m            B                           less than                               (                                                            λ                  A                                                  λ                  B                                                      )                    3                                    (        3        )            
where mA and mB are magnifications of the correcting optical system at the selected peak wavelengths xcexA and xcexB.
Further, it is preferable that the correcting optical system satisfies the following condition (4):                                           m            A                                m            B                          =                                            λ              A                                      λ              B                                .                                    (        4        )            
There are two ways to vary the magnifications in accordance with the wavelength.
In the first way, when the laser beam emitted from a multiline laser source is divided into a plurality of monochromatic beams, the correcting optical system comprises a plurality of optical elements that are disposed in optical paths of the respective monochromatic beams. The optical elements are different in a focal length such that the magnifications for the respective monochromatic beams are different from one another.
In the second way, when the laser beam having different wavelengths forms a single beam spot, the correcting optical system is arranged in the common optical path to cause chromatic aberration such that magnifications for the respective wavelengths are different from one another. In the second way, it is preferable that the correcting optical system is provided with a diffraction surface.
When the laser optical system of the invention is applied to a scanning optical system such as a laser photo plotter, a polygon mirror is arranged between the correcting optical system and the imaging optical system to deflect an incident beam.